Studies of twins
Studies of identical twins have repeatedly shown that sexual orientation is not determined genetically. Identical twins with identical genes do not always have the same orientation. If one is homosexual, the other is far more likely to be heterosexual. Even though statistically, compared with the general population, there is an increased chance that the twin will be homosexual, this is far from guaranteed. So though there may be a genetic ‘tendency’ toward homosexuality, sexual orientation is not genetically ‘determined’.